1. Field of the Invention
The present invention relates to a resin having a high refractive index that is useful for preparing plastic lenses, lenses comprising the resin and processes for preparing the same.
2. Description of the Prior Art
Plastic lenses have been rapidly popularized in recent years for use in optical elements such as lenses of eyeglasses and cameras. Plastic lenses are lightweight, less fragile than inorganic lenses and are capable of being dyed with ease.
A resin currently widely used for producing lenses for optical elements is a radical polymerization product of diethylene glycol bisallyl carbonate (herein-after abbreviated as DAC). The resin has excellent impact resistance, is lightweight, has prominent dye-affinity and good machinability including cutting ability and polishing ability.
However, lenses prepared from the resin have a smaller refractive index (n.sub.D =1.50) than glass lenses (n.sub.D =1.52). In order to obtain equivalent optical properties to glass lenses, it is necessary to increase the center thickness, peripheral thickness and curvature of the lens and hence the lens as a whole becomes very thick. Therefore, resins having a higher refractive index that are useful for making lenses are desired.
Resins having higher refractive indices that have been used to produce plastic lenses include polyurethane base resins obtained by reacting isocyanate compounds with hydroxyl compounds such as diethylene glycol (disclosed in Japanese Patent Laid-Open No. 136601/1982, U.S. Pat. No. 4,443,588), with halogen containing hydroxyl compounds such as tetrabromobisphenol-A (Japanese Patent Laid-Open No. 164615/1983) and with diphenyl sulfide linkage containing hydroxyl compounds (Japanese Patent Laid-Open No. 194401/1985).
The present inventors have previously invented resins having high refractive indices that are useful for preparing lenses by reacting isocyanate compounds with sulfur containing hydroxyl compounds (Japanese Patent Laid-Open No. 217229/1985, U.S. Pat. No. 4,680,369 and U.S. Pat. No. 4,780,522) or with polythiol compounds (Japanese Patent Laid-Open No. 199016/1985, U.S. Pat. No. 4,689,378, Japanese Patent Laid-Open Nos. 267316/1987 and 46213/1988, and U.S. Pat. No. 4,775,733). The present inventors prepared plastic lenses from their polyurethane base resins.
In molding the polyurethane base lens, it is usually difficult to release the polymerized lens from the mold due to adhesion between the polyurethane compounds and the mold. In order to improve the mold releasing property, the present inventors previously proposed a process in which an external mold releasing agent is used (Japanese Patent Laid-Open No. 267316/1987) or a process in which a polyolefin resin mold is used (Japanese Patent Laid-Open No. 236818/1987) to improve the mold release properties in preparing lenses.
Although the refractive indices of these lenses are higher than those of lenses prepared from DAC, the maximum refractive index of these lenses is between approximately 1.65 and 1.68. Further improvement of the refractive index is desired.
The above methods are still insufficient for improving the mold releasing property in casting polymerization of polyurethane base lenses.
In prior art processes in which external mold releasing agents are used, a portion of the agent that is applied to the inner surface of a mold migrates to the surface and interior of the polymerized lens which results in problems such as nonuniformity of the surface of the lens and turbidity of the lens. When a mold is used repeatedly, mold releasing treatment is required in each molding cycle. Such treatment lowers productivity in producing the lens and thus is extremely uneconomical in industrial production.
When the polyolefin resin molds are applied, the resin molds are deformed at elevated temperatures which causes large profile irregularities on the molded lens. Thus, the resin molds cannot be applied to articles requiring high accuracy on the molded surface.